High thermal conductivity metal matrix composites

ABSTRACT

Discontinuous diamond particulate containing metal matrix composites of high thermal conductivity and methods for producing these composites are provided. The manufacturing method includes producing a thin reaction formed and diffusion bonded functionally graded interactive SiC surface layer on diamond particles. The interactive surface converted SiC coated diamond particles are then disposed into a mold and between the particles and permitted to rapidly solidify under pressure. The surface conversion interactive SiC coating on the diamond particles achieves minimal interface thermal resistance with the metal matrix which translates into good mechanical strength and stiffness of the composites and facilitates near theoretical thermal conductivity levels to be attained in the composite. Secondary working of the diamond metal composite can be performed for producing thin sheet product.

The invention relates, in general, to very high thermal conductivitymetal matrix composite structures; and, more particularly, to suchcomposite structures containing diamond particles which have a thinconversion surface layer of beta-SiC formed thereon by a chemical vaporreaction process. The SiC coating on these particles is present as areaction zone, or graded layer, as opposed to a distinct SiC/coatingbuild-up without a diffusion conversion interface, that would have beenobtained by a coating process, such as chemical vapor deposition of SiCupon the diamond surface.

BACKGROUND OF THE INVENTION

There has been a considerable body of literature reported describingattempts to produce composite materials with very high thermalconductivity, e.g., for use in high power electronic packaging and otherthermal management applications. Much of this literature relates to theaddition of particulate fillers to a metal matrix, forming a metalmatrix composite (MMC). The benefit of adding a particulate filler witha high thermal conductivity to a metal to form a metal matrix compositeis well known. Properties of the MMC can often be optimized to suit therequirements of a particular application by properly selecting theproperties of the particulate filler and metal matrix. Examples wouldinclude the addition of SiC particles to an Aluminum matrix. The SiC isreadily wet by molten aluminum and aluminum alloys when it comes incontact with the filler particles. The Al/SiC composites have beenreported to achieve strengths of 400 MPa at a filler loading of >40 vol.%, indicating a good bond was formed between the SiC particles and Al.MMC's comprised of particulate SiC and an aluminum matrix haveadvantages over pure Al structures in terms of coefficient of thermalexpansion (CTE), stiffness, and wear resistance.

In general, however, the thermal conductivity of the Al/SiC MMC's do notmeet desired expectations. Thermal conductivity of pure aluminum is˜200W/m.k, and the thermal conductivity of pure crystalline SiCparticles is ˜320W/m.k. Values of thermal conductivity for Al/SiC MMC'sare generally <200W/m.k, and typically <180 W/m.k (ref 1-5). These AlSiCMMC's were consolidated by processes such as stir casting, powdermetallurgy, or low pressure and pressureless melt infiltration. Thesemethods are relatively slow, and have a considerable residence time whenthe aluminum is in the molten state, allowing the SiC to react with themolten Al forming aluminum carbide. An example reaction is:Al+3SiC=Al₄C₃+3Si   (1)For that reason, these processes generally require the use of Al—Sialloys which decrease the activity of the Si and reduce the kinetics ofthe adverse carbide reaction during long contact times with the moltenaluminum. These Al—Si alloys generally have a lower thermal conductivitythan pure aluminum, thus reducing the thermal conductivity of the SiC/AlMMC. Alternatively, the use of rapid high pressure metal infiltration(also referred to as squeeze casting) to consolidate the particulatereinforced aluminum composites results in a much faster consolidation ofthe composite. Exposure times of the particles to the molten aluminumare generally seconds as opposed to hours for the non-pressure processesdescribed above. As a result of the rapid consolidation with squeezecasting, pure aluminum can be used, and thermal conductivities of up toabout 225 W/m.k would be expected for SiC loadings of ˜55 vol. % in thecomposite.

Certain properties of diamond make it particularly attractive as apossible filler for high thermal conductivity MMC's. The thermalconductivity of diamond is about 700-2000 W/m.k, depending oncrystalline perfection. It also has a low CTE (approximately 1p.p.m./degree centigrade). However, researchers using consolidationprocesses for diamond/aluminum MMC's with a long exposure time fordiamond contact with the molten aluminum have been unable to obtain highlevels of thermal conductivity. Composites comprising an aluminum matrixcontaining 50 vol. % of industrial diamond particles have been reportedto have a thermal conductivity <200 W/m.k (Johnson and Sonuparlac, ref3). A microstructural examination of the diamonds in the compositerevealed the presence of a thick surface layer of aluminum carbide(Al₄C₃) on the diamond particles. This surface layer is formed by thereaction shown in equation (2).3C+4Al=Al₄C₃   (2)Aluminum carbide is generally recognized to have low thermalconductivity, and is hydroscopic. The diamond particles with the thicklayer of aluminum carbide formed on the surface, in effect, functionmore as an aluminum carbide particle than a diamond particle, resultingin poor thermal conductivity for the composite.

Coating the diamond particles with a protective layer before contactingthe diamond particles with molten aluminum forming the aluminumcomposite can prevent the reaction to form Al₄C₃. The application of adistinct SiC coating on diamond particles and subsequent compositeformation with Al has been described in the literature (ref 6). A loosebed of industrial diamond powder (Beta Diamond Products), with aparticle size of 40-50 microns, was coated with SiC using a chemicalvapor deposition process of the diamond particle array, which was termedchemical vapor infiltration, or CVI, by the authors. In this CVIprocess, a distinct SiC coating is applied or deposited on the surfaceof the diamond particles. (It is known in the art that the deposition ofSiC by the CVI process occurs at about 1000 degrees centigrade.) Johnsonand Sonuparlac estimated the thickness of the SiC coating varied between0.41 to 1.6 microns, depending on process conditions. They furtherestimated the total SiC content of the coated diamond particles at 3% to11% by volume. The preform particle arrays were observed to havestiffened by the CVI SiC coating. The SiC coated diamond preforms wereinfiltrated by a pressureless metal infiltration process, using anAl-15Si-5Mg wt. % alloy. Process conditions were optimized to assurecomplete infiltration of the preform. The relevant properties of theMMC's are shown in Table I. TABLE I Physical properties of ˜50 vol. %diamond/Al MMC composites described in reference 6. Coating Al₄C₃Thermal Young's Thickness content Density Conductivity modulus (Microns)(wt %) (gm/cc) (W/m-K) CTE (ppm/K) (Mpa) 0.41 0.078 3.168 239 6.8 3680.53 0.071 3.161 242 6.5 385 0.97 0.053 3.13 259 5.2 407 1.21 0.0473.125 131 5.9 408 1.23 0.073 3.22 240 4.6 398 1.42 0.12 3.213 225 5.0413 1.6 0.093 3.16 234 4.5 427

At all levels of SiC coating thickness, it appears the formation ofAl₄C₃ on the diamond particle has been reduced to a very low level, butremains greater than zero. The densities reported are consistent withthe author's claim of full infiltration of the SiC coated diamondpreforms with the aluminum alloys. The thermal conductivity values ofthe composite, ranging from 131-259 W/m.k, however, are very lowconsidering the relatively high loading of diamond particles in thecomposite (40-50 vol. %), and the thermal conductivity of the diamond.There is no apparent relationship between SiC coating thickness on thediamond particles and the thermal conductivity of the composite. Theauthors of that work indicated they believe that the increased stiffness(Young's modulus) observed with increased SiC coating thickness on thediamond particles is due to the formation of SiC bridges between thediamond particles. These results seem to indicate that the SiC coatingthickness may be excessive, thereby causing bridging between the diamondparticles. However, with the CVI process, it is difficult to obtain athin uniform SiC coating on the diamond particles, i.e. covering 100% ofthe diamond surface.

Unlike the above described prior art, the instant inventors have usedrapid high pressure melt infiltration (squeeze casting) to preparecomposites of aluminum with uncoated diamonds. As described above, thisprocess reduces exposure time of the diamond to the molten aluminum to avery short time (e.g., <2 seconds). Thermal conductivity of thecomposite was still <200 W/m.k, even though there was expected to beminimal aluminum carbide at the surface of the diamond particles.

BRIEF SUMMARY AND OBJECTS OF THE INVENTION

Briefly stated, the present invention involves the provision of acomposite structure comprised of a metal matrix having diamond particlesdispersed therein, wherein the diamond particles are characterized bythe presence of a layer of beta- SiC chemically bonded to the surfacethereof and wherein the carbon of the SiC is derived from the diamond ofthe respective particle to which it is bonded.

Advantageously, the matrix of the composite structure is comprisedessentially of a metal selected from among aluminum, copper,magnesiumand alloy of one or more of said metals.

In a preferred embodiment of the invention the matrix is comprised ofaluminum. In another embodiment, the matrix metal is comprised ofcopper.

The present invention is concerned with the preparation of diamondparticulate with SiC chemically bonded thereto that is an essentialcomponent of such novel metal matrix composites.

The instant invention further describes the formation of a metal matrixcomposite containing such diamond particles coated with SiC applied by adifferent process that results in a very high thermal conductivity forthe composite.

It is, therefore, a primary object of the present invention to provide ametal matrix composite with substantially improved thermal conductivity.

It is another valuable object of the present invention to provide a SiCcoated diamond particulate with the necessary characteristics to enableits use in the formation of such high thermal conductivity MMC's.

It is another valuable object of the present invention to provide anovel process to produce the above-mentioned SiC coated diamondparticulates.

It is another valuable object of the present invention to provide aprocess to incorporate the above-mentioned SiC coated diamondparticulate in a metal matrix and obtain the aforementioned very highthermal conductivity metal matrix composite.

Advantageously, the above-mentioned high thermal, conductivity metalmatrix compositions are produced by a method wherein the layer ofchemically bonded SiC is produced in-situ on the diamond particles of anarray thereof that is then embedded in the metal matrix, preferably by arapid high pressure metal infiltration technique known as squeezecasting.

Preferably, the chemically bonded layer of SiC is produced on therespective diamond particles by a chemical vapor relation process (CVR)by contacting the diamond particles with SiO gas.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of apparatus suitable to carry out the CVR processto coat diamond particles with SiC in accordance with the presentinvention.

FIG. 2 is a schematic of apparatus suitable to perform the squeezecasting assembly to form the diamond metal matrix composites inaccordance with the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

Referring now to FIG. 1, there is shown a schematic drawing of anapparatus suitable for preparing diamond particles for use as acomponent of a composite structure including a metal matrix in which thediamond particles are dispersed according to the present invention,wherein the prepared diamond particles have a conversion surface layerof beta-SiC formed thereon.

In FIG. 1 there is shown in side elevation a cross sectional view of acrucible 101 formed of SiC and which is divided into a lower chamber 102and an upper chamber 103 by means of a lower ring 104 of Si and an upperring 105 of SiC and having a web 106 of 100% SiC fabric disposed betweenthe two rings. The 100% SiC fabric was formed by reacting graphitefabric with gaseous SiO, to produce essentially 100% conversion of thegraphite to SiC.

The lower chamber 102 houses a SiO generator.

The SiO generator was prepared by mixing silicon (Si) and silica (SiO₂)in equimolar ratios. As the crucible 101 is heated above 1200 degreescentigrade, SiO gas is formed from the reaction in the generator asshown in equation 3:Si+SiO₂=2SiO (g)   (3)The SiO gas produced in the lower chamber 102 passes through the SiCfabric 106 to the upper chamber 103 and reacts with an array of diamondparticles 107 that are deployed on top of the SiC fabric 106 that asufficient quantity of SiO is generated to ensure the surface of thediamond particles is converted to SiC over the entire surface of eachparticle.

FIG. 2 shows a die assembly suitable for high pressure squeeze castingof metal matrix composites made in accordance with the presentinvention. This apparatus is fabricated from tool steel and consists ofthe die 110, die plug 111, and shot tube (or gate) 112. A cavity 113 ismachined in the die corresponding to the required geometry for thesqueeze cast part 113 a. The die plug 111 has a 0.005″ clearance to thedie cavity 113 to allow air to be vented from the casting as it fillswith molten aluminum. The inside diameter (ID) of the shot tube 112 issufficiently large such that it completely covers the die cavity 113. Toproduce a composite casting with aluminum and a particulate powder,. thepowder is placed in the die cavity 113, and shaken to provide a uniformloading of maximum density. Ceramic paper 114 is placed in the shot tube112 to cover the powder filled die 110. A quantity of molten aluminumsufficient to fill the die cavity 113 plus part of the shot tube 112 isthen poured in the shot tube 112. Pressure is then applied, up to 15,000psi via the plunger 115 to achieve a rapid filling of the die cavity 113and achieve approximately 100% density in the metal matrix composite.After cooling, the solidified part 113 a and partially filled shot tube112 containing the biscuit 115 are removed from the die assembly. Thebiscuit 115 is removed by metal removal techniques such as milling orsawing to produce the desired composite article or cast part 113 a.

For certain kinds of solid state devices, the trend in electronicpackaging is toward managing smaller devices with higher power levels.Management of very high power densities in many electronic chips hasbecome a major challenge. Lack of proper thermal management can causechip temperatures to increase during operation to the point where theyoverheat resulting in both performance and reliability reductions. Thepreferred route to prevent this heat buildup is to mount the chip on ahigh thermal conductivity substrate which will conduct the heat awayfrom the chip to a large heatsink therefore limiting the maximumtemperature of the chip during operation to an acceptable value. Thechip is generally attached to the substrate by means of a high thermalconductivity solder or some polymeric adhesive. Solder is generallypreferred over the adhesives because the solders generally have a higherthermal conductivity than the organic adhesive.

There is an additional mechanism by which the chip can be damaged, andthat relates to the relative coefficient of thermal expansion of thechip and the substrate. When a chip is attached to a substrate withsolder, the components must be heated to a temperature sufficiently highto melt the solder (180 degrees centigrade-425 degrees centigrade). Whenthe package is subsequently cooled, the solder solidifies providing abond between the chip and substrate. As the package is cooled further toroom temperature, differential thermal expansion of the chip andsubstrate can lead to the buildup of substantial mechanical stresses inthe chip. This cool down, plus continued thermal cycling of the packagecan cause fatigue in either the substrate, chip, or interface, affectingperformance and lifetime. When these stresses exceed the tensilestrength limits of the chip, they can cause cracking (delamination) tooccur. The materials commonly used for electronic chips are either Siwith a CTE ˜3 p.p.m./degrees centigrade, or GaAs which has a CTE ˜6p.p.m./degrees centigrade. It would be very desirable to use a substratewith a CTE that is closely matched to that of the chip. It would be evenmore desirable to have a substrate with such thermal expansioncharacteristics combined with a very high thermal conductivity.

One type of substrate used in the electronic packages described above isa ceramic substrate. For relatively low chip power levels, an aluminumoxide substrate is commonly used, which has a thermal conductivity of˜20-40 W/m.k and a CTE of ˜7 p.p.m./degrees centigrade. For higher powerchips, an aluminum nitride ceramic substrate with a thermal conductivityof ˜150-200 W/m.k and a CTE of ˜4 p.p.m./degrees centigrade may be abetter choice. Aluminum nitride ceramic substrates generally representthe highest thermal conductivity alternative in ceramic substrates otherthan the toxic BeO. These aluminum oxide and aluminum nitride ceramicsubstrates have excellent electrical insulating characteristics, whichis desirable for many electronic packages, including those based onbipolar chips. However, for electronic packages based on LDMOS chipswherein the bottom side of the device becomes the source contact, asubstrate is required which has good electrical conductivity between thefront and back faces or surfaces. While this may be accomplished byfabricating electrically conductive vias in the ceramic substrate, whichwould electrically connect those two surfaces, it would be generally besimpler to have the substrate body itself be electrically conductive, inaddition to having properties of high thermal conductivity and a goodCTE match to the chip. For these LDMOS devices, MMC substrates can oftenmeet the CTE and electrical conductivity requirements, but are limitedin the thermal conductivity they can achieve. The Al/SiC particulatereinforced MMC described above has a thermal conductivity of ˜180-225W/m.k, CTE of ˜7 p.p.m/degrees centigrade and good electricalconductivity. A much higher thermal conductivity would be desirablegiven the tendency for LDMOS chips to operate at very high powerdensities.

A metal matrix composite manufactured as described in the presentinvention with an aluminum matrix and containing particulate diamondprepared in accordance with the present invention can meet all the needsof those LDMOS electronic packages. Al has a thermal conductivity of˜200 W/m.k. Diamond has a thermal conductivity between about 700-2000W/m.k. Furthermore, the relatively low CTE of diamond (˜1 p.p.m./degreescentigrade) will result in a CTE for the composite which is a bettermatch to either Si or GaAs chips than the unfilled metals. However, withthe exception of the present invention, the particulate diamondreinforced MMC's described in the literature have demonstrated only amarginal improvement in thermal conductivity over the Al/SiC MMC'sreadily available. The MMC's described in the instant invention haveunexpectedly high thermal conductivities, with values up to ˜650 W/m.kdemonstrated to date, and with the potential for even higher levels ofthermal conductivity, e.g. values of 800 W/m.k and above.

Embodiment of FIG. 1

In order to make a metal matrix composite of Al with a diamondparticulate filler such that a very high thermal conductivity for theMMC is obtained, a thin, uniform, surface conversion, adherent coatingof SiC must be present on the diamond prior to composite formation. Thepurpose of this coating is to provide an interfacial coupling layerbetween the diamond and the aluminum, which in addition to being readilywet by the aluminum, provides graded acoustical impedance properties atthe interface for enhanced phonon transfer. Furthermore, in accordancewith the present invention, this SiC coating must be integrally bondedto the diamond in such a way that a distinct, discrete interfaceconversion layer between the diamond and the coating is produced toprevent any thermal barrier to heat transfer. Such a coating and processfor providing that coating is described in this invention. The preferredembodiment for this invention is a thin SiC conversion coating which isformed by a reaction between the diamond particles and a gaseous siliconspecies such as SiO, as described in equation 4:2C(diamond−s)+2 SiO(g)=SiC(s)+CO(g)   (4)It is believed that the CVR conversion of the diamond surface initiallyresults in a thin interface skin of SiC. (The inventors have observedformation of a thin interface surface skin of SiC in the reaction of SiOwith graphite flake.) As the reaction time and/or temperature areincreased, the coating development proceeds from the outer surface ofthe diamond particle toward the center, resulting in a reaction zone, orgraded layer. The coatings obtained by a CVI or CVD process, on theother hand, are expected to have an abrupt interface between the SiCcoating and the substrate. The CVI/CVD deposition temperature of SiC arebelow the temperature required to effect a conversion of the diamondsurface to SiC and results in only a built-up coating free of anyinterface reaction with an abrupt interface between the diamond andCVI/CVD SiC coating. It is believed this CVI interface contributed tothe poor thermal conductivity of the prior diamond/aluminum compositedescribed by Johnson and Sonuparlac in ref 6.

Furthermore, this chemical vapor reaction (CVR) process ensures that auniform surface layer of SiC is obtained at very low levels ofconversion coating thickness. As described above, as the SiC coatingthickness increases, the thermal conductivity of the SiC (˜320 W/m.k)acts as a thermal barrier between the very high thermal conductivitydiamond particles and the metal matrix, limiting the maximum thermalconductivity that can be achieved for the MMC.

In order to further understand the effectiveness of the thin SiC CVRconversion coating of the present invention, a prediction of compositethermal conductivity can be made using a rules of mixture calculation.While thermal conductivity is a transport property, and the rules ofmixture calculation is not exact, it can provide insight into acomparison of the CVI coating on diamond described by Johnson andSonuparlac in ref 6, with the CVR conversion coating of the presentinvention. The typical relation for simple mixtures would be:$\begin{matrix}{\left( {1 - f} \right)^{3} = {\frac{K_{in}}{K_{c}}\left\lbrack \frac{K_{c} - K_{p}}{K_{m} - K_{p}} \right\rbrack}^{3}} & (5)\end{matrix}$Where f is the volume fraction of particle, and K_(m), K_(p), and K_(c)are the thermal conductivities of the matrix, particle, and composite,respectively (ref 7). The above equation requires perfect heat transferacross the particle-matrix interface. This is limited by phononscattering at the interface between dissimilar components. Calculationswere carried out to estimate Kp in order to judge the effectiveness ofthe thermal interface. For an aluminum composite with 50 vol. % ofdiamond particles coated with the CVR process, a value of 200W/m.k wasused for Km, and 650 W/m.k for Kc (which was obtained for a compositeproduced using the CVR coating technology of this invention). Thisresulted in a calculated value for Kp of ˜1938 W/m.k which is close tothe maximum reported value for the thermal conductivity of diamond. Thisresult indicates the CVR coating as taught by the present inventionprovides a nearly perfect thermal interface. Next the calculation wascarried out for the composite described by Johnson and Sonuparlac in ref6, which had a thick CVI coating of the present invention on the diamondparticles. Using values of 45 vol. % for the diamond loading, 200 W/m.kfor Km, and 250 for Kc, the calculated value for Kp was ˜322 W/m.k. Thusthe thin, graded CVR coating provides a much better thermal interfacethan the CVI coating.

The role of the SiC in acoustic coupling can be better understood by thefollowing discussion. Treating phonons according to wave theory, andassuming transverse acoustic waves at normal incidence, the (intensity)reflection coefficient at a boundary between two different materials isgiven as: $\begin{matrix}{R = \left\lbrack \frac{Z_{1} - Z_{2}}{Z_{1} + Z_{2}} \right\rbrack^{2}} & (6)\end{matrix}$Where Z is the acoustic impedance, equal to the square root of theproduct of mass density and elastic modulus. Clearly, minimumreflection, and therefore maximum transmission, occurs when Z₁=Z₂Reflection, and therefore phonon scattering, increases as delta Zincreases. In the case of thermal conductivity, the optimum acousticimpedance for a coating material would be: Z_(coat)={square root}{squareroot over (Z₁Z₂)} (7)

Table II gives the acoustic impedance of the relevant materials. Notethat the acoustic impedance of SiC is close to the optimum matchingimpedance calculated from Equation 7 (276×10⁵ kg/m.s calculated vs.310×10⁵ kg/m.s for SiC) for an Al-diamond interface. This would predictthe excellent performance of SiC conversion coated diamond in thealuminum matrix. TABLE II Acoustic Impedance of Various Materials AlDiamond SiC Z(10⁵ kg/ms): 136 561 310

A preferred process to coat the particulate diamond powder isillustrated in FIG. 1. The diamond particles are placed on a supportingSiC fabric, which is then placed in a crucible and heated to thereaction temperature. The crucible is placed in a furnace, in which aSiO partial pressure is generated by a reaction between SiO₂ and areducing agent such as Si or C. After a predetermined time attemperature for the SiO-diamond conversion reaction to be completed, thefurnace is cooled, and the fabric supporting the coated diamonds removedfrom the crucible. The SiC conversion coated diamond particles areremoved from the fabric by peeling off the fabric from the coateddiamonds. Any agglomerates are then broken up by mechanical agitation.

The reaction between the diamond and SiO to form SiC coated diamond willgenerally proceed more rapidly as the process temperature is increased.However at a certain temperature threshold, the diamond can be convertedto graphite. In order to establish the limiting conditions in theapparatus in FIG. 1, the following experiments were conducted. Diamondparticulate (Synthetic Diamond from Oshmens Corp.) was placed in acrucible and heated to 1550 degrees centigrade and 1600 degreescentigrade for up to 4 hours. X-ray diffraction (XRD) of the heattreated powders indicated the pattern for diamond, with no evidence ofgraphite. In a separate experiment, diamond powder, which had beenreacted with SiO (Table II, Experiment 4), was heated to 1660 degreescentigrade for 3 hours. The XRD pattern gave no indication of anyConversion of diamond to graphite. However, when the same powder washeated to 1800 degrees centigrade for 8 hours, the XRD pattern on thissample showed a large graphite peak, a broadened diamond peak, and a SiCpeak from the SiC conversion reaction. The significant conversion ofdiamond to graphite after 8 hours at 1800 degrees centrigrade led to adecision to restrict the diamond-SiO reactions to lower time and/ortemperature.

A series of furnace runs were then carried out to react the diamondparticulate with SiO. The SiO(g) generator for the conversion of diamondparticles was made by adding equimolar ratios of silicon (Si) and silica(SiO₂). The silica used had a particle size of 150-300 microns. Themixture was milled for 1 hour using alumina milling media. The reactionof the Si and Sio₂ proceeds faster as the particle size of the Sio₂ isreduced. To minimize this effect, silica of the same particle size(150-300 microns) was used and the milling time was fixed at 1 hour.

Diamond powder of varying particle size, from 0.5 microns to 120microns, was reacted at temperatures ranging from 1450 degreescentigrade to 1600 degrees centigrade for times of 1-8 hours. The coateddiamond particles were then examined by XRD. The XRD pattern for diamondis shown in FIG. 3, and the pattern for beta-SiC in FIG. 4. Theexperiments are summarized in Table II. The XRD spectra of the diamondsshowed a change that is attributed to the presence of a smallconcentration of SiC. FIG. 5 is an example of the XRD of such a coatingrun, example 13 from Table III, wherein diamond powder with a 100-120micron particle size was reacted with SiO for 3 hours at 1550 degreescentigrade. TABLE III Experimental Runs to Prepare SiC Coated Diamondsvia Reaction with SiO Diamond Time at Ratio of XRD Particle Maximum Max.Peak Height Expt. # Size (microns) Temperature(C.) Temp(hr) SiC/diamond1 15-30 1450 1 0.026 2 15-30 1550 3 0.073 3 120-150 1550 3 0.017 4100-120 1550 3 0.014 5 30-40 1550 3 0.064 6 40-50 1550 3 0.014 7 100-1201550 3 0.023 8 100-120 1550 3 0.015 9 100-120 1600 4 0.035 10 100-1201600 4 0.034 11 100-120 1450 8 0.032 12 100-120 1550 3 0.041 13 100-1201550 3 0.033 14 0.5-2.0 1550 3 0.595 15 0.5-2.0 1450 3 0.138 16 0.5-2.01450 1 0.111 17 0.5-2.0 1420 3 0.103

The last column of Table III shows the ratio of the peak height forbeta-SiC at 2theta=35.7 to the peak height for diamond at 2theta=44.0.While the ratio of peak heights from XRD is not a quantitative procedurefor compositional analysis, it allows a rough estimate of relative (butnot absolute) SiC to diamond content in the reacted powders. The data inTable II indicates the variable with the largest impact on SiC formationis the particle size of the diamond. The peak height ratio of SiC todiamond is generally 0.01-0.04 for 100-120 micron diamond particles,0.03-0.07 for 15-30 micron diamond, and 0.1-0.6 for 0.5 -2 microndiamond. Thus the largest diamond particles appear to have the lowestSiC content. For a given particle size diamond, increasing thetemperature and time for the reaction generally results in a higherratio of SiC peak height to diamond peak height. Given this information,and the supposition of this invention, it would be expected that thehighest thermal conductivity for a composite would be obtained using thediamonds which have the lowest ratio of SiC XRD peak height to diamondXRD peak height. One such diamond powder would be from Experiment 4,Table III, with a particle size of 100-120 microns, and reacted for 3hours at 1550 degrees centigrade with SiO. Conversely, the diamondpowder with a particle size of 0.5-2.0 microns could be expected toproduce composites with the lowest thermal conductivity.

Embodiment of FIG. 2.

FIG. 2 is a schematic of the squeeze casting assembly to form thediamond metal matrix composites. Diamond powder is placed directly intothe die cavity of a tool steel die and shaken to achieve full tapdensity. The diamond powder bed is covered with a sheet of {fraction(1/32)}″ alumina ceramic paper that acts as a filter to remove oxideparticles from the melt during infiltration. To avoid entrapment ofgases in the die casting, an 0.005″ gap is maintained around the bottomplug in the die to allow trapped air to escape during infiltration.Aluminum is heated above the melt temperature (660 degrees centigrade)under Argon cover gas to avoid oxidation. The die is heated separatelyto a temperature slightly below the melting point of the Al. The heateddie is removed from the heating furnace and transferred into the castingmachine and Al melt is poured into the heated shot tube of the die.Infiltration is completed within a few seconds under pressurization. Thedie is cooled by unforced cooling in the casting machine prior toremoval of the casting, which consists of the cast part and attachedbiscuit.

Other Embodiments

It is to be understood that while the invention has been described inconjunction with the detailed description thereof, the foregoingdescription is intended to illustrate and not limit the scope of theinvention, which is defined by the scope of the appended claims. Otheraspects, advantages, and modifications are within the scope of thefollowing claims.

EXAMPLES EXAMPLE 1

An aluminum metal matrix composite having diamond particles dispersedtherein was formed by the following method. Loose uncoated diamondpowder (Synthetic Diamond from Oshmens Corp.) of size 15-30 microns wasplaced directly into the die cavity of a tool steel die ( as illustratedin FIG. 2) and shaken to achieve full tap density of 50-55%. Dimensionsof the die cavity were 2″×2″×0.25″ The diamond powder bed was coveredwith a sheet of {fraction (1/32)}″ thick high alumina ceramic paperwhich acts as a filter to remove oxide particles from the melt duringinfiltration. To avoid entrapment of gases in the die casting, a 0.005″gap was maintained around the bottom plug in the die to allow trappedair to escape during infiltration. Aluminum metal with a purity of 99.8%was heated to 850 degrees centigrade under Ar cover gas to avoidoxidation. The die was heated separately to a temperature of 650 degreescentigrade, slightly below the melting point of the Al (660 degreescentigrade). The heated die was removed from the heating furnace andtransferred into the casting machine within 2 seconds and the Al meltwas poured into the heated shot tube of the die maintained at 200degrees centigrade. Infiltration was completed with 2 seconds, using aram speed of 0.1 inch/sec. under pressurization and a maximum pressureof 15 ksi . The die was cooled to 300 degrees centigrade by unforcedcooling in the casting machine prior to removal of the casting. Laserflash measurement of thermal conductivity on a number of samplesproduced by a number of casting runs returned values of conductivity inthe range of 55-60 W/m.k. Measurement of strength of the composite using4-point bend rupture yielded an average rupture strength of 22 ksi(average of 6 measurements).

EXAMPLE 2

The procedure for forming an aluminum metal matrix composite withuncoated diamond powder described in example 1 was repeated usinguncoated 100-120 micron diamond powder.

Laser flash measurement of thermal conductivity on a number of samplesproduced by a number of casting runs returned values of conductivity inthe range of 150-200 W/m.k. Measurement of strength of the compositeusing 4-point bend rupture yielded an average rupture strength of 17 ksi(average of 6 measurements).

EXAMPLE 3

The procedure for forming an aluminum metal matrix composite withuncoated diamond powder described in example I was repeated usinguncoated 0.5-2 micron diamond powder.

Laser flash measurement of thermal conductivity on a number of samplesproduced by a number of casting runs gave an average value ofconductivity of 105 W/m.k. Measurement of strength of the compositeusing 4-point bend rupture yielded an average rupture strength of 22 ksi

EXAMPLE 4

A process for producing an array of diamond particles having aconversion coating of SiC on the respective particles was performed asfollowed. Diamond particles (Synthetic Diamond from Oshmens Corp.) witha particle size of 15-30 microns were reacted with SiO by a chemicalvapor reaction process as follows. Crucibles of the description shown inFIG. 1 were utilized. The crucibles were loaded with 30 grams of diamondparticles, and the crucibles were placed in a high temperature furnacewhere SiO was caused to flow through the array of diamond particles incontact with the surface of each particle and the particles were heatedin the furnace. The furnace was heated to 1550 degrees centigrade in anon oxidizing atmosphere using a flowing argon atmosphere, and held atthat temperature for 3 hours. After the furnace was cooled to roomtemperature, the diamond particles with the conversion coating of SiCwere removed from the SiC fabric.

EXAMPLE 5

The coated diamond powder comprising an array of conversion coateddiamond particles from example 4 (15-30 micron particle size) was placeddirectly into the die cavity of a tool steel die (as illustrated in FIG.2) and shaken to achieve full tap density of 50-55%. Dimensions of thedie cavity were 2″×2″×0.25″ The diamond powder bed of SiC conversioncoated particles was covered with a sheet of {fraction (1/32)}″ thickhigh alumina ceramic paper which acts as a filter to remove oxideparticles from the melt during infiltration. To avoid entrapment ofgases in the die casting, a 0.005″ gap was maintained around the bottomplug in the die to allow trapped air to escape during infiltration.Aluminum metal with a purity of 99.8% was heated to 850 degreescentigrade in a non-oxidizing atmosphere, under Ar cover gas to avoidoxidation. The die was heated separately to a temperature of 650 degreescentigrade, slightly below the melting point of the Al (660 degreescentigrade). The heated die was removed from the heating furnace andtransferred into the casting machine within 2 seconds and the Al meltwas poured into the heated shot tube of the die maintained at 200degrees centigrade. Infiltration was completed within 2 seconds, using aram speed of 0.1 inch/sec under pressurization and a maximum pressure of15 ksi. The die was cooled to 300 degrees centigrade by unforced coolingin the casting machine prior to removal of the casting. Laser Flashmeasurement of thermal conductivity on a number of samples produced by anumber of casting runs gave values of conductivity in the range of234-250 W/m.k. Measurement of strength of the resulting composite using4-point bend rupture yielded average rupture strength of 54 ksi (averageof 6 measurements).

EXAMPLE 6

Diamond particles of size 100-120 microns were reacted with SiO byrepeating the chemical vapor reaction process described in example 4 toproduce an array of SiC conversion coated diamond particles of largerparticle size than those of Example 4.

EXAMPLE 7

The die casting procedure for forming an aluminum metal matrix compositedescribed in example 1 was repeated using the 100-120 micron chemicalvapor reaction treated diamond powder array described in example 6.Laser flash measurement of thermal conductivity on a number of samplesof the composite of SiC conversion coated diamond particles in thealuminum metal matrix produced by a number of casting runs gave valuesof conductivity in the range of 575-620 W/m.k. Measurement of strengthof the composite using 4-point bend rupture yielded an average rupturestrength of 51 ksi (average of 6 measurements).The coefficient ofthermal expansion for the composite was measured at 7.1×10⁻⁶in/in/degrees centigrade.

EXAMPLE 8

Diamond particles of size 0.5-2 microns were reacted with SiO by thechemical vapor reaction process described in example 4 to produce anarray of conversion coated diamond particles of different size.

EXAMPLE 9

The die casting procedure for forming an aluminum metal matrix compositedescribed in example 1 was repeated using the array of 0.5-2 microndiamond particle chemical vapor reaction treated diamond powderdescribed in example 8. Laser flash measurement of thermal conductivityon a number of samples of the composite of SiC conversion coated diamondparticles in the aluminum matrix produced by a number of casting runsgave an average value of conductivity of 150 W/m.k. Measurement ofstrength of the composite using 4-point bend rupture yielded an averagerupture strength of 51 ksi.

EXAMPLE 10

One gram of phenolic resin was mixed with 1.2 grams of Si powder andball milled overnight. Four grams of diamond powder with a particle sizeof 100-120 microns was added to the mixture, which was thoroughly mixed.The mixture was pressed at 5 ksi pressure in a 0.5″ diameter die andheated to 160 degrees centigrade to provide green strength. Theresulting small pellet was pyrolyzed at 1600 degrees centigrade for 1 hrto produce a rigid preform with strength of >15 ksi. The pellet wasplaced in a steel die and infiltrated with Al using the same conditionsas described above in example 1. Laser Flash measurement of thermalconductivity on a number of samples of the composite of SiC conversioncoated diamond particles in the aluminum matrix produced by a number ofcasting runs produced values of conductivity in the range of 300-320W/m.k.

EXAMPLE 11

The process of example 10 was repeated using a reduced amount ofphenolic and Si: 0.5 gm. of phenolic resin and 0.6 gm. of Si powder.Diamond powder weight was the same as in example 10 at 4 gm. Laser Flashmeasurement of thermal conductivity on a number of samples of thecomposite of SiC conversion coated diamond particles in the aluminummatrix produced by a number of casting runs produced an average value ofconductivity of 475 W/m.k.

EXAMPLE 12

A composite structure comprising diamond particle dispersed in amagnesium metal matrix was formed using a process generally as describedin example 1. Loose uncoated diamond powder with a particle size of100-120 microns was placed directly into the die cavity of a tool steeldie of dimensions 2″×2″×0.25″ and shaken to achieve full tap density of50-55% The diamond powder bed was then covered by a sheet of {fraction(1/32)}″ thick high alumina ceramic paper which acts as a filter toremove oxide particles from the melt during infiltration. To avoidentrapment of gases in the die casting, a 0.005″ gap was maintainedaround the bottom plug in the die to allow trapped air to escape duringinfiltration. The 99.8% purity Mg melt was heated to 830 degreescentigrade under Ar-2.5% SF6 cover gas to avoid oxidation. The die washeated separately to a temperature of 620 degrees centigrade, slightlybelow the melting point of the Mg (647 degrees centigrade ). The heateddie containing the diamond powder was removed from the heating furnaceand transferred into the casting machine within 2 seconds and pure Mgmelt was poured into the heated shot tube of the die maintained at 200degrees centigrade. Infiltration was completed within 2 seconds, using aram speed of 0.1 inch/sec. under pressurization and a maximum pressureof 15 ksi. The die was cooled to 300 degrees centigrade by unforcedcooling in the casting machine prior to removal of the casting. Laserflash measurement of thermal conductivity on a number of samples of thecomposite produced by a number of casting runs returned values ofconductivity in the range of 120-250 W/m.k.

EXAMPLE 13

The procedure for forming a magnesium metal matrix composite asdescribed in example 12 was repeated using the 100-120 micron chemicalvapor reaction treated diamond powder having an integral conversioncoating of SiC as described in example 6. Laser flash measurement ofthermal conductivity on a number of samples of the composite of SiCconversion coated diamond particles dispersed in the magnesium metalmatrix produced by a number of casting runs returned values ofconductivity in the range of 520-550 W/m.k. Measurement of strength ofthe composite using 4-point bend rupture yielded an average rupturestrength of 28 ksi (average of 6 measurements).

EXAMPLE 14

A quantity of diamond particles with a size of 100-200 microns wereplaced in a column with an argon gas purge which was pulsed in order tojolt and move the diamond particles. The column of diamond particles washeated to 1200 degrees centigrade and a SiC organometallic (many aresuitable for depositing SiC such as, methyltrichloro silane,dimethyldichlorosilane, triethylsilane, etc.) gas was passed through thediamond particles such that a thin coating of SiC was deposited on thediamond particles. After deposition stopped, the quantity of SiC coateddiamond particles was heated to 1600 degrees centigrade for 1 hour tocause diffusion of the SiC into the diamond surface and produce aconversion coating of SiC on the diamond particles, which were thensuitable for inclusion as the dispersed diamond particles in a matrix ofmagnesium metal as in example 13.

EXAMPLE 15

An analogous experiment to example 14 using a magnesium metal matrix wasperformed except silicon tetrachloride and hydrogen was utilized todeposit silicon on the diamond (particle size 100-120 microns) particlesurface at 1100 degrees centigrade. After a thin, coating of silicon wasformed, the Si coated diamond was heated to 1600 degrees centigrade for1 hour, wherein the Si reacted with the diamond surface to form aconversion layer of SiC on the diamond particle surface.

EXAMPLE 16

The SiC conversion coated diamond particles described in examples 14 and15 were squeeze cast with aluminum as described in example 1 to producea composite SiC coated diamond particles in an aluminum metal matrix. Alaser flash measurement of thermal conductivity gave values in the rangeof 550-620 W/m.k.

EXAMPLE 17

To make net shape parts, graphite tooling for a casting die was preparedfrom graphite stock which was premachined with a shaped cavitycorresponding to the shape and thickness of the required net shape partand covered with a graphite lid. Tooling tolerances were maintained at±0.005″ to plates and round parts. The completed graphite tooling andlid was then perforated with a series of {fraction (1/16)}″ holesdrilled through it to allow air to escape from the diamond filler duringcasting. The graphite tooling could also be patterned with a series ofgrooves to correspond to the fins on a heat sink design for instance orwith a stud at a location corresponding to a through hole in a heatspreader diamond/Al part to allow an attachment point. To fill the diewith diamond powder, the graphite tooling was wrapped in ceramic paperwhich was fixed in place by continuous graphite fiber in a hoop windingconfiguration to cover all the through drilled holes. The diamond powderwas poured into the mold and tapped to full tap density corresponding to45-55% of the volume of the mold. The graphite tooling was placed in asteel permanent pressure casting die and infiltrated with moltenaluminum metal at a pressure of 3-10 ksi. To remove the graphite toolingafter casting, the overlying Al metal was machined away and the graphitetooling surrounding the fully infiltrated composite piece was removed bygrit blasting with 325 mesh glass beads. The remaining {fraction(1/16)}″ metal feeders or gates corresponding to the holes drilledthrough the original tooling to facilitate Al flow were sheared awayusing a hand tool to leave a net shape composite diamond-aluminumcasting. Thermal conductivity measurement on composite diamond-aluminumstructures in the form of net shape parts cast in the graphite toolingusing diamond powder as described in example 6, wherein the particlesize of the diamond was 100-120 microns and the powder had a CVR SiCconversion coating, gave values in the range of 520-600 W/m.k. Fourpoint bend strength of the castings was 46-51 ksi. In one heat spreaderapplication, the gate tabs were left attached such that this geometryprovided a high surface area to provide for exceptionally high heattransfer with the passing of air over the tabs.

EXAMPLE 18

A diamond/Al composite plate of dimensions 2×2×0.1″ was made by themethod of graphite tooling described above in example 17 and was hotrolled at 550 degrees centigrade to a final thickness of 0.05″, usingmultiple roll passes of 0.010″ thickness reduction per pass with crossrolling after every other pass to maintain flatness of the part. Someminor edge cracking was observed after rolling which extended 0.1″ intothe part. These edges were removed by shearing. Thermal conductivity ofthe part was measured before and after rolling and remained unchanged at520 W/m.k. Strength of the piece was 51 ksi measured in 4-point bending.

EXAMPLE 19

A diamond/Al composite plate of 0.05″ thickness was produced by therolling operation described above in example 18. This composite platewas hot stamped in a closed die at 500 degrees centigrade to form a0.125×0.25×0.03″ thickness piece with a raised rim of 0.05″ heightsurrounding it. Thermal conductivity of the piece was maintained at 520W/m.k after stamping.

EXAMPLE 20

A diamond/Al composite plate of size 1×1×0.2″ was produced by squeezecasting as described above in example 7. This composite plate was thenopen die hot forged at 500 degrees centigrade with applied pressure of15 ksi at a strain rate of 0.0001/sec. After forging, the thickness ofthe composite plate was reduced from 0.25 to 0.05″ which corresponded toa forging strain of 150%. Measurement of thermal conductivity of thecomposite plate after forging showed that the thermal conductivity ofthe material remained unchanged from the starting value of 585 W/m.kbefore forging.

EXAMPLE 21

100-120 mesh size (100 micron) coated diamond powder, as described inexample 6, was fully mixed with 99.8% purity Al powder of 45 micron sizeby tumbling in a ceramic ball mill. The weight ratio of the two powderswas chosen to correspond to a 40% by volume diamond/Al composite. Thepowder mixture was transferred into a 1″ diameter casting graphite dieand hot pressed at a temperature close to the Al melting point for 1 hrat 640 degrees centigrade at a pressure of 6 ksi. The resultingcomposite diamond and aluminum billet was sectioned for strength andthermal conductivity measurement. Thermal conductivity measurements werein the range of 400-430 W/m.k and four point strength measurements were46-52 ksi.

EXAMPLE 22

A composite structure comprised of a metal matrix composed of copperhaving SiC conversion coated diamond particles dispersed therein wasformed by the following methods. Loose SiC conversion coated diamondpowder of 100-120 micron size prepared as in example 6 was well mixedwith 99.9% purity Cu powder to produce a Vf=35-40% diamond loading. Thepowder was tumbled for 2 hrs to complete mixing and ‘canned’ in anevacuated Mo tube and hipped at a pressure of 15 ksi, for 2 hrs at atemperature of 1025 degrees centigrade. The consolidated billet wasremoved from the ‘canning’ and warm forged to a thickness reduction of75%. The composite material was sectioned for thermal conductivitymeasurement and showed a value of 550 W/m.k.

EXAMPLE 23

A similar composite structure to that described in example 22 comprisedof a copper metal matrix with diamond particles dispersed therein wasformed as follows: Loose SiC conversion coated diamond powder of 100-120microns size was copper metallized using known art of chemical reductionand electrolyte. The coating thickness was sufficient to provide aVf=35-40% diamond particle loading on full powder consolidation. Thecoated powder was consolidated using field activated sinteringtechniques (FAST) known in the art as plasma activated sintering (PAS)which consists of applying a pressure to the powder (copper coateddiamond with the SiC surface conversion) during a pulsed discharge ofenergy. The thermal conductivity was determined to be 566 W/m.k.

Reference

1) “Packaged for the Road” Mechanical Engineering, July 2001.

2) Lanxide Corporation published data sheets of composites fabricated byLANXIDE TM, PRIMEX TM and PRIMEX CAST TM, USA, 1997.

3) Duralcan Corp. published data sheets of composites fabricated by stircasting, USA, 1994.

4) A. L. Geiger, D. P. H. Hasselman, and P. Welch, Acta Mater., Vol 45,No. 9, pp 3911-3914, (1997).

5) “A new substrate for Electronic packaging: AlSiC Composites”, M.Occhinero, R. Adams, and K. Fennessey, Proceedings of the 4^(th) AnnualPortable by Design Conference, Electronics Design, March 24-27, pp398-403.

6) “Diamond/Al metal matrix composites formed by the pressureless metalinfiltration process”, W. Johnson and B Sonuparalak, J. MaterialsResearch, V8, No. 5, pp. 1169-1173, 1993.

7) “The effect of particle size on the thermal conductivity ofZnS/diamond composites”, A. Every, Y. Tzou, D. Hasselman, R. Raj, ActaMater., V40, No.1, pp. 123-129, 1992.

1) A composite structure comprised of a metal matrix having diamondparticles dispersed therein, wherein said diamond particles arecharacterized by the presence of a layer of beta-SiC chemically bondedto the surface thereof. 2) The structure of claim 1, wherein the metalmatrix is comprised essentially of a metal selected from aluminum,copper, magnesium, and alloys of one or more of said metals. 3) Thestructure of claim 1 wherein the metal matrix material is essentiallyaluminum. 4) The structure of claim 1 wherein the metal matrix materialis essentially copper. 5) A method of forming a composite structurecomprised essentially of a metal matrix having diamond particlesdispersed therein, and wherein said diamond particles are characterizedby the presence thereon of a surface layer of beta-SiC chemically bondedthereto, comprising the steps of contacting the surface of said diamondparticles with a source of Si under conditions whereby the Si ischemically reacted with C comprising the surface of the diamondparticles, thereby forming said surface layer of beta-SiC bonded to saiddiamond particles, wherein said composite structure is formed byassembling a plurality of said diamond particles in an array, andencapsulating the particles of said array with said metal in a moltenstate whereby the particles of said array bearing the surface layer ofbeta-SiC are dispersed in a matrix of said metal when the molten metalhardens upon cooling to the solid state. 6) The method of claim 5wherein the surface layer of beta-SiC is formed on said diamondparticles by contacting said diamond particles with the source of Siprior to assembling said particles in said array. 7) The method of claim5 wherein the surface layer of beta-SiC is formed on said diamondparticles by contacting said diamond particles with the source of Sisubsequent to assembling said particles in said array. 8) The method ofclaim 5 wherein the array of diamond particles is encapsulated by themolten metal in the performance of a squeeze casting process to formsaid composite structure. 9) The method of claim 6 wherein the array ofdiamond particles is encapsulated by the molten metal in the performanceof a squeeze casting process to form said composite structure. 10) Themethod of claim 7 wherein the array of diamond particles is encapsulatedby the molten metal in the performance of a squeeze casting process toform said composite structure. 11) The method of claim 6, wherein the Sithat contacts the diamond particles is provided by SiO gas that iscaused to contact the diamond particles. 12) The method of claim 7,wherein the Si that contacts the diamond particles is provided by SiOgas that is introduced into the array of diamond particles. 13) Themethod of claim 6 wherein the Si that contacts the diamond particlesincludes Si powder. 14) The method of claim 7 wherein the Si thatcontacts the diamond particles includes Si powder. 15) The method ofclaim 6 wherein the Si powder is mixed with phenolic resin. 16) Themethod of claim 7 wherein the Si powder is mixed with phenolic resin.17) The structure of claim 1 wherein the carbon of the SiC is derivedfrom the respective diamond particles to which it is bonded. 18) Thestructure of claim 1 wherein the diamond particles are in the size rangeof 100 - 120 microns. 19) An array of diamond particles characterized bythe presence of a layer of beta SiC chemically bonded to the surface ofthe respective particles and whereon the carbon of the SiC is derivedfrom the diamond of the respective diamond particles to which it isbonded. 20) A composite consisting of a powder comprising diamondparticles, wherein the surface of the respective particles is convertedto a thin SiC surface conversion coating, such coating being formed by achemical vapor reaction of a gaseous Si source, with the respectivediamond particle, and said SiC coated diamond powder being suitablybound in a metal matrix producing a composite with a thermalconductivity higher than the matrix. 21) A process to coat diamondpowder with a thin SiC surface coating, wherein the diamond particlescomprising the diamond powder are contacted by a gaseous Si containingspecies under conditions causing chemical vapor reaction between thegaseous Si containing species and the solid particles comprising thediamond powder, said SiC coated diamond powder being suitable to beencapsulated in a metal matrix to form a very high thermal conductivitymetal matrix composite. 22) The process described in claim 21, whereinthe gaseous species of Si is generated by the reduction of SiO₂ to formgaseous SiO. 23) A process to coat the particles comprising a diamondpowder with a thin, in-situ SiC coating obtained by depositing a Sicontaining preceramic polymer layer on the diamond particle that can beconverted to SiC by heating to a suitably high temperature to initiatethe conversion of the diamond surface to SiC, but at a time insufficientto convert the diamond structure to graphite, said SiC coated diamondpowder being suitable to form very high thermal conductivity metalmatrix composites. 24) The process described in claim 23, wherein theconversion is carried out at a temperature of at least about 1500degrees centigrade. 25) The process described in claim 23, wherein theconversion is carried out at a temperature of at least about 1600degrees centigrade. 26) The process described in claim 23, wherein theconversion is carried out at a temperature of at least about 1800degrees centigrade. 27) The process described in claim 23, wherein thereaction time at maximum temperature is less than about 1 hour. 28) Theprocess described in claim 23, wherein the reaction time at maximumtemperature is less than about ½ hour. 29) The process described inclaim 23, wherein the reaction time at maximum temperature is less thanabout ⅛ hour. 30) The process described in claim 23, wherein thereaction time at maximum temperature is less than about {fraction(1/10)} hour. 31) A process to coat the particles comprising a diamondpowder with a thin SiC surface conversion coating, wherein the coatingis obtained by passing a reactive gaseous mixture over the diamondparticles, such that the reactive gas produces a silicon containingspecies at a temperature that causes reaction between the siliconcontaining species and the diamond, but at a time and temperatureinsufficient to convert the diamond structure to graphite, said SiCpowder being suitable to form very high thermal matrix composites. 32)The process described in claim 31, wherein the reaction is carried outat a temperature of at least about 1100 degrees centigrade. 33) Theprocess described in claim 31, wherein the reaction is carried out at atemperature of at least about 1500 degrees centigrade. 34) The processdescribed in claim 31, wherein the reaction is carried out at atemperature of at least about 1800 degrees centigrade. 35) The processdescribed in claim 31, wherein the reaction time at maximum temperatureis less than about 1 hour. 36) The process described in claim 31,wherein the reaction time at maximum temperature is less than about ½hour. 37) The process described in claim 31, wherein the reaction timeat maximum temperature is less than about ⅛ hour. 38) The processdescribed in claim 31, wherein the reaction time at maximum temperatureis less than about {fraction (1/10)} hour. 39) A composite consisting ofdiamond particles, wherein the surface of the particles is converted toa thin SiC surface conversion coating, such coating being formed byheating a preform of Si powder and diamond powder plus a binder, suchas, but not limited to, phenolic resin, the purpose of said binder beingto hold the Si in contact with the diamond surface, heating said preformto a temperature sufficient to cause the surface conversion reaction tooccur, and said SiC conversion coated diamond powder being suitablybound in a metal matrix producing a composite with a thermalconductivity higher than the matrix. 40) A process to coat the particlescompressing a diamond powder with a thin SiC surface coating, wherein apreform of phenolic resin, Si powder, and diamond powder are heated to atemperature sufficient to cause the surface conversion reaction tooccur, said SiC coated diamond powder being suitable to form very highthermal conductivity metal matrix composites. 41) The compositedescribed in claim 39, wherein the temperature to which the preform isheated is at least 1300 degrees centigrade. 42) The composite describedin claim 39, wherein the temperature to which the preform is heated isat least 1400 degrees centigrade. 43) The composite described in claim39, wherein the temperature to which the preform is heated is at least1500 degrees centigrade. 44) The composite described in claim 39,wherein the temperature to which the preform is heated is at least 1600degrees centigrade. 45) The composite described in claim 39, wherein thetemperature to which the preform is heated is at least 1700 degreescentigrade. 46) The process described in claim 40, wherein thetemperature to which the preform is heated is at least 1300 degreescentigrade. 47) The process described in claim 40, wherein thetemperature to which the preform is heated is at least 1400 degreescentigrade. 48) The process described in claim 40, wherein thetemperature to which the preform is heated is at least 1500 degreescentigrade. 49) The process described in claim 40, wherein thetemperature to which the preform is heated is at least 1600 degreescentigrade. 50) The process described in claim 41, wherein thetemperature to which the preform is heated is at least 1700 degreescentigrade. 51) A metal matrix composite comprising a matrix containingdiamond powder, said diamond powder having a thin SiC surface layer onthe respective diamond particles comprising said powder, said SiC layersbeing comprised of a conversion coating formed by a chemical vaporreaction of SiO with the respective diamond particles, and said metalmatrix composite having a thermal conductivity greater than about 300W/m.k. 52) The composite described in claim 51, having a thermalconductivity greater than about 400 W/m.k. 53) The composite describedin claim 51, having a thermal conductivity greater than about 500 W/m.k.54) The composite described in claim 51, having a thermal conductivitygreater than about 600 W/m.k. 55) The composite of claim 51, wherein themetal employed is aluminum. 56) The composite of claim 52, wherein themetal employed is aluminum. 57) The composite of claim 53, wherein themetal employed is aluminum. 58) The composite of claim 54, wherein themetal employed is aluminum. 59) The composite of claim 51, wherein themetal employed is magnesium. 60) The composite of claim 52, wherein themetal employed is magnesium. 61) The composite of claim 53, wherein themetal employed is magnesium. 62) The composite of claim 54, wherein themetal employed is magnesium. 63) The composite of claim 51, wherein themetal employed is copper. 64) The composite of claim 52, wherein themetal employed is copper. 65) The composite of claim 53, wherein themetal employed is copper. 66) The composite of claim 54, wherein themetal employed is copper. 67) The composite described in claim 51,wherein the content of the CVR SiC coated diamond powder in thecomposite is about 10-60 vol % of the total. 68) The composite describedin claim 51, wherein the content of the CVR SiC coated diamond powder inthe composite is greater than about 70 vol % of the total. 69) Thecomposite described in claim 51, wherein the content of the CVR SiCcoated diamond powder in the composite is greater than 80 vol % of thetotal. 70) The composite described in claim 51, wherein the particlesize of the diamond powder used is preferably about 50-150 microns, andmore preferably 100-120 microns. 71) The composite described in claim51, wherein the particle size of the diamond powder used is preferablygreater than 150 microns. 72) The composite described in claim 51,wherein the particle size of the diamond powder is greater than about200 microns. 73) The composite described in claim 51, wherein theparticle size of the diamond powder is greater than about 300 microns.74) A metal matrix composite containing diamond powder, said diamondpowder comprising diamond particles having a thin SiC surface conversionlayer formed thereon, such layer being formed by heating a preform of Sipowder and diamond powder plus a binder, such as, but not limited to,phenolic resin, the purpose of said binder being to hold the Si incontact with the surface of the respective diamond particles, heatingsaid preform to a temperature sufficient to cause the surface conversionreaction to occur, and said metal matrix composite having a thermalconductivity greater than about 300W/m.k. 75) The composite described inclaim 74, having a thermal conductivity greater than about 400 W/m.k.76) The composite described in claim 74, having a thermal conductivitygreater than about 500 W/m.k. 77) The composite described in claim 74,having a thermal conductivity greater than about 600 W/m.k. 78) Thecomposite described in claim 74, wherein metal comprising the matrix isaluminum 79) The composite described in claim 75, wherein the metalcomprising the matrix is aluminum. 80) The composite described in claim76, wherein the metal comprising the matrix is aluminum. 81) Thecomposite described in claim 77, wherein the metal comprising the matrixis aluminum. 82) The composite described in claim 74, wherein the metalcomprising the matrix is magnesium. 83) The composite described in claim75, wherein the metal comprising the matrix is magnesium. 84) Thecomposite described in claim 76, wherein the metal comprising the matrixis magnesium. 85) The composite described in claim 72, wherein thematrix metal is copper. 86) The composite described in claim 74, whereinthe matrix metal is copper. 87) The composite described in claim 75,wherein the matrix metal is copper. 88) The composite described in claim76, wherein the matrix metal is copper. 89) The composite described inclaim 77 wherein the matrix metal is copper. 90) The composite describedin claim 74, wherein the content of the CVR SiC coated diamond powder inthe composite is about 10-60 vol % of the total. 91) The compositedescribed in claim 74, wherein the content of the CVR SiC coated diamondpowder in the composite is greater than about 70 vol % of the total. 92)The composite described in claim 74, wherein the content of the CVR SiCcoated diamond powder in the composite is about greater than about 80vol % of the total. 93) The composite described in claim 74, wherein theparticle size of the diamond powder used is preferably about 50-150microns. 94) The composite described in claim 74, wherein the particlesize of the diamond powder used is greater than about 150 microns. 95)The composite described in claim 74, wherein the particle size of thediamond powder used is greater than about 200 microns. 96) The compositedescribed in claim 74, wherein the particle size of the diamond powderused is greater than about 300 microns. 97) A process for making a metalmatrix composite with very high thermal conductivity, said compositecomprising of metal matrix containing a powder formed of diamondparticles dispersed therein, comprising the steps of: (a) providing therespective diamond particles comprising the diamond powder with a thinlayer of SiC by contacting the said diamond particles with a gaseous Sicontaining species while subjecting said particles to heat at atemperature sufficient to cause the Si of the gas to react with thesurface carbon of the diamond particles thereby converting the surfacelayer of the claimed particles to a thin layer of SiC, (b) placing theSiC layered particles comprising of the diamond powder in the die cavitysqueeze casting apparatus (c) covering the diamond powder with a sheetof alumina ceramic paper, (d) introducing by pressure squeeze casting amolten metal selected to form the matrix into the die containing thecoated diamond powder, under conditions appropriate to pressureinfiltrate the diamond powder with the molten metal, and cooling thedie. 98) The process of claim 97 wherein the metal employed for themetal matrix is aluminum. 99) The process of claim 97 wherein the metalemployed for the metal matrix is magnesium. 100) The process of claim 97wherein the metal employed for the metal matrix is copper. 101) Aprocess for making a metal matrix composite with very high thermalconductivity, said composite comprising a metal matrix containing apowder formed of diamond particles dispersed therein, comprising thesteps of: (a) providing the respective particles comprising the selecteddiamond powder with a thin layer of SiC by preparing a mixture ofphenolic resin, Si powder and said diamond powder, pressing said mixtureto form a preform, and heating said preform to a temperature sufficientto cause a chemical reaction between the Si powder and the carbonsurfaces of the respective diamond particles which results in theformation of a thin SiC surface conversion layer on the respectivediamond particles, (b) placing the SiC layered diamond particles in thedie cavity of a squeeze casting apparatus, (c) and covering the diamondpowder with a sheet of alumina ceramic paper, (d) introducing moltenmetal selected to form the matrix into said die cavity under conditionsof heat and pressure effective to pressure squeeze cast the molten metalin the die containing the SiC layered diamond powder, therefor pressureinfiltrating the diamond powder with the molten metal, and cooling thedie. 102) The process of claim 101 wherein the metal employed for themetal matrix is aluminum. 103) The process of claim 101 wherein themetal employed for the metal matrix is magnesium. 104) The process ofclaim 101 wherein the metal employed for the metal matrix is copper.105) A process for making a metal matrix composite with very highthermal conductivity, said composite comprising a metal matrixcontaining a powder formed of diamond particles dispersed therein,comprising the steps of: (a) providing the respective particlescomprising the diamond powder with a thin surface layer of SiC byreacting the carbon surfaces of said diamond particles with a gaseous Sicontaining species at a temperature sufficient to form SiC, (b) forminga homogeneous mixture of the above SiC layered diamond particles withmetal powder selected to form the matrix, and (c) forming a densecomposite by hot pressing the above mixture of SiC surface layereddiamond particles and metal powder at a temperature, pressure and cycletime sufficient to fully densify the composite. 106) A process formaking a metal matrix composite with very high thermal conductivity,said composite comprising a metal matrix containing a powder formed ofdiamond particles dispersed therein, comprising the steps of: (a)coating the respective particles comprising the diamond powder with athin layer of SiC by reacting said diamond particles with a gaseous SiCcontaining species at a temperature sufficient to form SiC, (b) forminga homogeneous mixture of the above SiC coated powder with metal powder,and (c) forming a dense composite by hot isostatic pressing the abovemixture of SiC coated diamond powder and metal powder at a temperature,pressure and cycle time sufficient to fully densify the composite. 107)A high thermal conductivity substrate for LDMOS electronic packages,wherein the substrate is a metal matrix composite comprising a metalcontaining dispersed therein diamond particulates which have a thin SiCcoating, said coating having been produced by a chemical vapor reactionprocess between gaseous SiO and the respective diamond particles. 108)The substrate of claim 107, wherein the metal employed for the metalmatrix is aluminum. 109) The substrate of claim 107, wherein the metalemployed for the metal matrix is magnesium. 110) The substrate of claim107, wherein the metal employed for the metal matrix is copper. 111) Thesubstrate of claim 107, wherein the process utilized to consolidate themetal matrix composite is pressure squeeze casting. 112) The substrateof claim 107, wherein the metal employed for the metal matrix isaluminum. 113) The substrate of claim 107, wherein the metal employedfor the metal matrix is magnesium. 114) The substrate of claim 107,wherein the metal employed for the metal matrix is copper. 115) Anelectronic package containing an LDMOS chip bonded to a substrate withvery high thermal conductivity, wherein said substrate is a metal matrixcomposite containing particulate diamond, and said particulate diamondhas a thin SiC surface coating formed on the respective particlesthereof by a chemical vapor reaction process between gaseous SiO andsaid diamond powder. 116) The electronic package described in claim 115,wherein the metal employed for the metal matrix is aluminum. 117) Theelectronic package described in claim 115, wherein the metal employedfor the metal matrix is copper. 118) The electronic package described inclaim 115, wherein the process utilized to consolidate the metal matrixcomposite is pressure squeeze casting. 119) The electronic packagedescribed in claim 115, wherein the process utilized to consolidate themetal matrix composite is pressure squeeze casting. 120) The electronicpackage described in claim 96, wherein the process utilized toconsolidate the metal matrix composite is pressure squeeze casting. 121)A high thermal conductivity substrate for LDMOS electronic packages,wherein the substrate is a metal matrix composite containing diamondparticulates which have a thin SiC conversion coating, such coatingbeing formed by heating a preform of Si powder and diamond powder plus abinder, such as, but not limited to, phenolic resin, the purpose of saidbinder being to hold the Si in contact with the diamond surface, andheating said preform to a temperature sufficient to cause the surfaceconversion reaction to occur. 122) The substrate of claim 121, whereinthe metal employed for the metal matrix is aluminum. 123) The substrateof claim 121, wherein the metal employed for the metal matrix ismagnesium. 124) The substrate of claim 121, wherein the metal employedfor the metal matrix is copper. 125) The substrate of claim 121, whereinthe process utilized to consolidate the metal matrix composite ispressure squeeze casting. 126) The substrate of claim 121, wherein themetal employed for the metal matrix is aluminum. 127) The substrate ofclaim 121, wherein the metal employed for the metal matrix is magnesium.128) The substrate of claim 121, wherein the metal employed for themetal matrix is copper. 129) An electronic package containing an LDMOSchip bonded to a substrate with very high thermal conductivity, whereinsaid substrate is a metal matrix composite containing diamondparticulates which have a thin SiC conversion coating, said coatingbeing formed by heating a preform of Si powder and diamond powder plus abinder, such as, but not limited to, phenolic resin, the purpose of saidbinder being to hold the Si in contact with the diamond surface, andheating said preform to a temperature sufficient to cause the surfaceconversion reaction to occur 130) The electronic package described inclaim 129, wherein the metal employed for the metal matrix is aluminum.131) The electronic package described in claim 129, wherein the metalemployed for the metal matrix is copper. 132) The electronic packagedescribed in claim 129, wherein the process utilized to consolidate themetal matrix composite is pressure squeeze casting. 133) The electronicpackage described in claim 129, wherein the process utilized toconsolidate the metal matrix composite is pressure squeeze casting. 134)The electronic package described in claim 129, wherein the processutilized to consolidate the metal matrix composite is pressure squeezecasting. 135) An electronic package for high power applications,requiring very high thermal dissipation, comprising a compositestructure as defined in claim 1 having a high thermal conductivityfurther characterized by the presence of a layer of aluminum nitridedeposited on the surface thereof, said aluminum nitride providing anelectrically insulating layer and also providing a high thermalconductivity path allowing for electronic chips to be bonded to theelectrically insulating aluminum nitride layer, and also allowingelectronic circuitry to be formed or deposited on the aluminum nitride.136) A method of forming a composite structure comprised essentially ofa metal matrix having diamond particles dispersed therein, and whereinsaid diamond particles are characterized by the presence thereon of asurface layer of beta-SiC chemically bonded thereto, comprising thesteps of contacting the surface of said diamond particles with a sourceof SiC under conditions whereby the SiC is deposited on the surface ofthe diamond particles, causing the deposited SiC to diffuse into thediamond particle on which it is deposited by subjecting to heat at atemperature and for a time adequate to cause such diffusion, therebyforming said surface layer of beta-SiC bonded to said diamond particles,and forming said composite structure by encapsulating a plurity of saidparticles in array with said metal in a molten state, whereby theparticles of said array bearing the surface layer of beta-SiC aredispersed in a matrix of said metal when the molten metal hardens uponcooling to the solid state. 137) The method of claim 136 wherein thesource of SiC is a SiC organometallic gas. 138) The method of claim 137wherein the SiC organometallic gas is selected from amongmethyltrichloro silane, dimethyldichlorosilane, and triethylsilane. 139)The method of claim 138 wherein the array of diamond particles areenclosed in a non oxidizing atmosphere prior to being contacted by theSiC organometallic gas and whereas the diamond particles are heated toabout 1600 degrees centigrade for about one (1) hour to cause the SiC todiffuse.